1. Field of the Invention
The present invention relates to a developing method and, more particularly, to a developing method of visualizing an electrostatic latent image formed on an image retainer in electrophotography.
2. Description of the Prior Art
As a developing apparatus to be used for the above-specified development, the following apparatus is widely adopted because its size reduction is feasible. More specifically, a developing sleeve acting as a developer feeding member has its surface formed of a magnetic brush of a magnetic developer by the action of a magnet disposed at the back thereof, and the magnetic developer is fed to a developing zone to apply a toner under a developing bias to an electrostatic latent image on a image retainer.
In recent years, the demands for increasing the reproducing speed and density of the copies of an electrophotographic reproducing apparatus have become more and more intense to make it accordingly desirable to speed up the movement of the image retainer thereby to assure development of the electrostatic latent image within a short period. On the other hand, the promoted trend of coloring papers in offices and so on has enhanced the need for reproducing colored hard copies. It has also been desired to provide a developing method which is appropriate for realizing a color reproducing apparatus having a high resolution and an excellent color reproducibility.
In order to satisfy the desire for speeding up the development, it is conceivable to use a developing apparatus which is equipped with a plurality of developing sleeves. This apparatus will enlarge the developing apparatus to lose the aforementioned merits.
The developer is generally divided into a one-component developer composed of a magnetic toner and a two-component developer composed of a non-magnetic toner and a magnetic carrier. The latter two-component developer is appropriate for the color reproduction partly because it can obtain a toner image of clear color without any necessity for containing a black or brown magnetic component in the toner and partly because it is feasible to control the charge of the toner. In a color reproducing method in which toner images of plural colors are formed and superposed on an image retainer (i.e., a photosensitive member), a non-contact developing method is appropriate, in which the development is conducted by keeping a magnetic brush out of contact with the image retainer so that the toner image or images previously developed may not be broken. The non-contact development is a method in which an a.c. and/or d.c. bias is applied to the developer feeding member to form an alternating electric field in a developing region, while the developer on its member being kept away from the image retainer, thereby to float the toner and attach on the electrostatic latent image.
As the developing method using the two-component developer in a non-contact or quasi-contact manner, there can be enumerated Japanese Patent Laid-Open Publication Nos. 56-144452, 57-139761, 59-67565, 59-91453, 59-121077, 59-154469 and 59-181362.
Unless the development is sufficient for color reproductions with color toners, the copies obtained are so badly reproducible that they provide poor appearances. In the color reproductions of the prior art, therefore, the magnetic brush is rotated at a high speed for the development. However, this development has a high rotational torque, leaves traces of the brush, and provides insufficient density. As a result, it is the current practice that copies of satisfactory image qualities cannot be obtained.
In the reversal development, in addition to the above-specified problems, there arises another troublesome problem that a developer charged with an opposite polarity to that of the electrostatic charges on the photosensitive member is liable to stick to the non-image portion (i.e., the white background), especially the non-image portion around the image portion (i.e., the colored portion) thereby to deteriorate the image quality due to fogging and to waste the development.